1. Field of the Invention
The invention relates to a communications technology using radio communications. In particular, the invention relates to a positioning apparatus, a positioning method, and a positioning system for determining the position of a mobile station such as a cellular phone.
The present application claims priority from Japanese Application No. 2001-051476, the disclosure of which is incorporated herein by reference for all purposes.
2. Description of the Related Art
In recent years, communication systems using radio communications start prevailing rapidly. Further developments are expected in the fields of mobile communications, such as cellular phones and pagers, and navigation systems for assisting persons and vehicles in moving.
For further progress of these communication systems, it is desired to develop positioning methods and the like which determine the position of a mobile station such as a movable cellular phone and navigation system, establish a stable communication state etc. with base stations according to the result of positioning, and also ensure adaptability to mobile communications environments and navigation system environments which are becoming diversified.
Among the conventional positioning methods is a positioning method for a mobile communications system of code division multiple access (CDMA) scheme.
In this positioning method, as shown in FIG. 25, a cellular phone, or mobile station P, receives radio waves emitted from base stations A, B, and C which are arranged within the communication area. The propagation time (or propagation ranges) required for the radio waves to reach the mobile station P from the respective base stations A, B, and C are determined, and a triangulation-based analysis is performed to determine the position of the mobile station P with respect to the base stations A, B, and C.
The conventional positioning method will be described more concretely. As shown in FIG. 26, the mobile station P contains a positioning apparatus which has a range measuring unit 6 and a position computing unit 7 connected to a receiving unit 1.
The mobile station P includes the receiving unit 1, a transmitting unit 2, an RF unit 3, and a transmitting/receiving antenna ANT for conducting communications with base stations. In the mobile station P, the transmitting/receiving antenna ANT receives the incoming waves from the base stations A, B, and C. Reception signals resulting from the reception are down-converted by the RF unit 3 before they are A/D converted into digital data. The digital data is passed through a roll-off filter 4 and despread by a demodulating unit 5 to generate reception data Drx. The range measuring unit 6 and the position computing unit 7 arranged in the mobile station P perform the triangulation-based analysis using output data Dd of the roll-off filter 4 and the reception data Drx of the demodulating unit 5 to determine the present position of the mobile station P.
As shown in FIG. 27, the range measuring unit 6 includes a correlator 8 and a range computing unit 9. The correlator 8 obtains correlations between correlating data DA, DB, and DC having correlativity to the incoming waves from the base stations A, B, and C, and the output data Dd from the roll-off filter 4, respectively. The range computing unit 9 analyzes correlation values CRRA, CRRB, and CRRC determined by the correlation operations to detect the propagation ranges LA, LB, and LC of the respective incoming waves.
More specifically, as shown in FIGS. 28(a)–28(c), the correlator 8 computes the correlation values CRRA, CRRB, and CRRC corresponding to the incoming waves from the base stations A, B, and C, respectively. The range computing unit 9 compares those correlation values CRRA, CRRB, and CRRC with a predetermined threshold value THD to peak-detect positions (points of time) tA, tB, and tC where the correlation values reach their maximums, respectively. Then, the phase differences τA, τB, and τC up to the respective points of time tA, tB, and tC are regarded as propagation time, and are converted into propagation ranges. Thereby, the propagation ranges LA, LB, and LC of the incoming waves are determined.
The position computing unit 7 subjects the propagation ranges LA, LB, and LC to the above-mentioned triangulation for analysis, and thereby determines the position of the cellular phone P. In other words, the base stations A, B, and Care supposed to transmit radio waves which contain information for indicating the positions (latitudes, longitudes) of the respective base stations A, B, and C. Upon reception, the position computing unit 7 extracts the positional information as to the base stations A, B, and C out of the reception data Drx. It analyzes the positional information extracted and the propagation ranges LA, LB, and LC through the application of the triangulation, and obtains positional data Dp which shows the present position of the mobile station P.
The conventional positioning method, however, has had the problems that the positioning precision may deteriorate due to the influence of multipath fading and noise, and that it is difficult to improve the positioning precision because of the susceptibility to multipath fading and noise.
To name a concrete example for description, as shown in FIG. 29, a building or suchlike shielding BL1 lies between, e.g., the base station A and the mobile station P. Here, the direct wave coming from the base station A to the mobile station P lowers in level accordingly. Besides, multipath waves reflected from other buildings or suchlike reflecting objects BL2 and BL3 shall arrive at the mobile station P. In such a case, as shown in FIG. 30(a), a plurality of peak values greater than the threshold value THD can appear in the correlation value CRRA, resulting from the direct wave and the multipath waves. This makes it impossible to determine which peak value is of the direct wave. This has caused the problem that a peak value occurring from a multipath wave can possibly misjudged to be of the direct wave.
In addition, the direct wave relatively decreases in level as compared to the multipath waves coming to the mobile station P because of the shielding BL. This has produced the problem that when the peak value resulting from the direct wave falls below the threshold value THD and a peak value resulting from a multipath wave exceeds the threshold value THD, as shown in FIG. 30(b), the phase difference (time) τAe up to the position of appearance of the peak value resulting from the multipath wave can be misjudged to be of the direct wave.
Such a condition as shown in FIG. 30(b) also occurs not only under the influence of multipath waves but also in cases where noise having correlativity to the correlating data DA for the base station A is received and the peak value resulting from the noise appears in the correlation value CRRA. Hence, there has been a problem of difficult distinction between noise and a direct wave.
In the presence of such problems, i.e., situations that the propagation time or propagation range of the direct wave coming from the base station A to the mobile station P is misjudged as the propagation time τAe or propagation range LAe resulting from a multipath wave or noise, then a position Pe off the original position (true position) of the mobile station P might be determined to be the present position as shown in FIG. 31. This has deteriorated the positioning precision.
The foregoing example has dealt with the case where the direct wave from the base station A cannot be detected with high precision. High precision detection may also fail on the direct waves from the other base stations B and C, due to the effects of multipath waves and the like. Thus, it has been difficult to improve the positioning precision.
That is, in the triangulation, the positions of the respective base stations A, B, and C are known. Therefore, as long as the propagation ranges LA, LB, and LC of the three direct waves from the base stations A, B, and C are precisely detectable, the true position of the mobile station P may be considered to fall at a single point where three circles intersect one another on the assumption that the three circles are drawn around the base stations A, B, and C with the propagation ranges LA, LB, and LC as the radii, respectively. Nevertheless, in actual communication environment, the propagation ranges of the incoming waves from the base stations A, B, and C to the mobile station P may vary with errors at random due to the influence of multipath fading and noise. It follows that various positions, e.g., those in the hatched region in FIG. 31 can be misjudged to be the true position of the mobile station P. This has caused a problem of difficult improvement in positioning precision.